The present invention relates to an apparatus for irradiating a focused ion beam (FIB) onto a device such as a semiconductor large scale integration (LSI) element and thereby conducting a fine machining work thereof and/or a fine film fabrication thereon, and in particular, to a processing apparatus using ion beam generating means and a processing method of the same in which when the focused ion beam is irradiated from an ion source onto a sample such as a silicon wafer or a silicon device, the sample is not contaminated with substances emitted from the ion source.
The focused ion beam (FIB) has been adopted in various apparatuses for various purposes such as a maskless ion implantation, an ion beam lithography, a mask modification, a wiring modification in the semiconductor manufacturing field, a secondary ion mass spectrometry (SIMS) in the analysis field; and operations to prepare samples to be observed, for example, creation of observation cross sections of samples for the scanning electron microscope (SEM) and production of thin-film pieces for the transmission electron microscope (TEM). In such apparatuses utilizing focused ion beams, there has been broadly employed a liquid metal ion source. Specifically, in the apparatuses to achieve the machining work and modification above, gallium has been usually put to practices as the ion element. Furthermore, in an analyzer which analyzes sample elements by milling, there have been commonly used noble gas ions and/or oxygen ions produced from an ion source of the duoplasmatron type.
On the other hand, to increase the production yield, it is quite important to carry out the following process control operations in the semiconductor manufacturing processes. Namely, in each of the processes such as ion implantation, lithography, and etching, a check is made to determine whether or not initial specifications thereof are fully satisfied and an investigation is conducted to decide, for example, whether or not a desired form to be actually obtained is deformed, for example, due to unexpected dirts and/or dusts. Moreover, in a case where such a defect exists, a correction process to remove the defect is effected after the pertinent process. To keep the sample free of any contamination from outside of the production line, the control operations are essentially required to be accomplished in the production line. That is, so-called inline observation, modification, and/or analysis are necessary for this purpose. In the present stage of art, a surface observation is achieved in an actual fabrication line by an optical microscope or a scanning electron microscope.
The beam size from the liquid metal ion source can be reduced to an order of submicron. The source is practically used in a focused ion beam a machining apparatus to mill a semiconductor LSI chip, a mask, etc. However, metal ions irradiated onto a sample surface to be milled by physical sputtering act, although there exist exceptions of some metal elements, as impurities in general, in the subsequent semiconductor production processes and possibly exert adverse influences on the semiconductor devices. Consequently, it is impossible at present to positively use such liquid metal ion sources in the semiconductor manufacturing line.
In this situation, to clarify the problems to be solved by the present invention, description will be given respectively of 1 problems in FIB application, 2 problems of ion sources to produce focused ion beams, 3 problems related to ion sources.
1 Problems in FIB Application
In contrast to the optical and scanning electron microscopes currently used in the semiconductor device production, there are not presently employed in the mass-production line such apparatuses utilizing a focused ion beams (FIB) as an FIB cross section machining apparatus to observe local cross sections of a device or an element, an FIB correcting and machining facility to modify process defects and logical defects of the device, and a secondary ion beam mass spectrometry (SIMS) apparatus adopting the FIB to conduct composition analyses of particular positions of the element. Namely, these apparatuses are used for a sample for a test or check taken out from the production line or a sample completely undergone the production processes.
That is, these apparatuses are used on the condition that the sample processed thereby is not returned to the production line again. This is because the conventional apparatuses using the focused ion beam contaminates the sample and/or the sample production line.
For example, when a Ga-FIB is used in the machining of cross sections of a silicon wafer or device and the SIMS analysis therefor, gallium atoms are accumulated on the silicon layer such that the piled gallium elements function as a p-type dopant (acceptor) for silicon elements. This leads to an electric deterioration of the product in a long period of time. Moreover, due to quite a low vapor pressure of gallium atoms, there appears a gallium deposition around the area of the wafer or device processed by the Ga-FIB. This exceeds a simple electric contamination, namely, a conductive layer is formed and hence there arises a significant problem of, for example, a short circuit between wirings in the device.
In addition, even when a local fine machining work is done by an Si-FIB produced from a liquid metal ion source (LMIS) adopting an Au--Si alloy, gold (Au) particles evaporated from the LMIS causes a heavy-metal contamination on the sample such as an Si wafer or device, which leads to an adverse effect in the device operation.
On the other hand, in the case of the duoplasmatron type, the plasma of a noble gas is produced so as to extract ions from the plasma into a focused ion beam. Consequently, ions irradiated onto a machining surface become gas atoms or molecules, after giving their momentum to the surface, and hence do not act as impurities any adverse influence directly in the semiconductor fabrication processes. However, due to the arc discharge to generate the plasma, a strong electric field applied onto the filament cathode and a high temperature thereof cause impurity metal ions to be mixed into the plasma. This leads to an unfavorable effect onto the semiconductor fabrication processes. In consequence, like in the case of the liquid metal ion source, the ion source of the duoplasmatron type cannot be used in the semiconductor fabrication line.
FIG. 2 shows an example of a process check of a semiconductor device employing the conventional FIB milling apparatus. In the semiconductor manufacturing processes, transistor elements and circuits are fabricated on the silicon wafer through such processes as film deposition, etching, and ion implantation. In a difficult process, for example, forming a film on a surface of deep holes or etching fine grooves, a check is necessary to confirm whether or not the process has been appropriately carried out. In this case, a wafer is obtained from the previous process line. At a position where a cross section thereof is desired to be observed for a check, a hole is milled by a FIB machine using Ga ions, thereby observing a side wall thereof by an SEM.
However, due to the Ga-FIB milling, irradiated gallium remains on the wafer and adversely influence the device as described above. In addition, when the wafer is returned to the production line, contamination is extended to various other manufacturing apparatuses of the line. There consequently exists a possibility of further contamination in wafers fabricated by such contaminated apparatuses. To avoid this problem, as shown in FIG. 2, the wafer milled by FIB for check is not returned to the production line. Namely, the wafer is discarded. However, there are about 100 to several hundred of LSI chips on a wafer having a diameter of 200 millimeters (mm), and much value has been added on each chip at this check point. Discard of the wafer thus having such an added value is quite undesirable for economic reasons. Moreover, the wafer diameter will be possibly increased to about 300 mm in the future; consequently, this problem cannot be any longer ignorable.
Moreover, when the process check using the FIB is conducted in an off-line stage as is the case in the conventional system, a wafer is removed from the line for each FIB milling or analysis process. Since several hundred chips are manufactured in a wafer, when the wafer is resultantly discarded, the other chips not undergone the work or analysis are wasted. This is also an economic problem to be solved.
FIG. 3 shows a graph of the production yield of semiconductor devices. In the processes of manufacturing semiconductor devices, probability of occurrences of fatal defects is presented by a fatal defect density per unit area .alpha. (number of defects per cm.sup.2). Experimentally, the defect density .alpha. decreases as the stage progresses from test production to mass production, to a value ranging from one defect to several defects. However, the value will not be reduced to 0. The number of fatal defects in a layer increases in proportion to the chip area. In other words, the yield .beta. of acceptable products with respect to a certain layer decreases as the chip area is increased as shown in FIG. 3. For example, when the chip area is 14 mm by 14 mm=1.96 cm.sup.2, the yield .beta. is attained as (1-1.96.alpha.).times.100%.
On the other hand, a semiconductor device consists of many layers. FIG. 4 shows a relationship of a chip yield and number of wiring layers on assumption that the yield .beta. is fixed for each layer. Only when each of n wiring layers is satisfactorily fabricated, an acceptable chip is obtained. The chip yield, therefore, is represented as .beta..sup.n. According to technological trends of semiconductor devices, the chip area is increasing; moreover, especially in a logic LSI chip, the total number of wiring layers is now in transition from four to six. Consequently, assuming that the fatal defect density .alpha. is fixed, the chip yield is tend to decrease. Accordingly, to obtain a favorable chip, it will be necessary in the future to correct or to modify defects caused thereon through the process layer by layer. In the modification, the FIB process is favorably used. However, as described above, once the Ga-FIB processing step is accomplished for the modification of the layer, the wafer cannot be brought back to the manufacturing process line. Namely, heretofore, the modification during the processes has not been feasible.
In consideration of the background art described above, there have been desired a working method using the FIB and an apparatus implementing the method capable of working a sample such as a wafer or a device in an inline operation without contaminating the sample.
An ion beam machining method of manufacturing a semiconductor device has been described in the JP-A-2-90520 entitled "Ion Beam Working Method" (known example 1). According to this example, the semiconductor device includes, particularly, a silicon substrate and the ion species is at lest one selected from a group including Si, C, Ge, Sn, and Sm elements. Particularly, according to an embodiment thereof, an Au--Si alloy (Au.sub.82 Si.sub.18) is adopted to produce silicon ions. Moreover, germanium and samarium ions are obtained from alloys of such elements as Al and Au.
In this regard, the liquid metal ion source (LMIS) and the electrohydrodynamic Ion source (EHDIS) are substantially identical to each other, namely, these sources have an identical configuration.
2 Problems of Ion Sources to Produce Focused Ion Beams
To work a silicon substrate or a silicon device by the FIB without causing any electric contamination thereon, it is obviously favorable to use Si and Ge ions because these ions do not have an energy level as impurity atoms in the forbidden energy band related to the silicon energy level. In addition, inert gas elements such as Ne, Ar, Kr, and Xe are also appropriate for the purpose.
According to quite a simplest method of producing a beam of silicon or germanium ions from an LMIS or EDHIS, an alloy containing silicon or germanium atoms is used as the ion material. The ion beam generating system is operated while lowering the melting point and the vapor pressure of the material. Only Si+ and Si.sup.2 + ions or only Ge+ and Ge.sup.2 + ions are gathered by an EXB mass spectrometer into a focused beam. The known silicon alloys include the Au--Si, Pt--Si, and Al--Si alloys; whereas, as for the known Ge alloys, there exist the Au--Ge, Fe--Ge, Pt--Ge, and Cu--Ge alloys.
As above, to emit silicon or germanium ions, it has been commonly known to employ an alloy in which silicon or germanium atoms are respectively mixed with Au, Cu, Fe, or Pt atoms. However, when such an alloy is adopted as the ion material, the silicon element to be worked is contaminated as above with the element (for example, Au, Cu, or Pt atoms) other than the silicon or germanium contained in the alloy. Namely, there cannot be achieved an examination free of contamination required for the inline examination. Particularly, it has been well known that the silicon semiconductor production line is essentially required to be free of heavy metal elements such as Au and Pt.
As described above, on the other hand, in the case of the ion source of the duoplasmatron type, the arc discharge is adopted to create a plasma, impurity metal ions are mixed into the plasma from the filament cathode due to the high electric field and high temperature applied to the cathode. This results in an adverse influence upon the semiconductor production line.
Resultantly, it is to be appreciated that in order to conduct the check with FIB free of contamination, it is necessary to use silicon or germanium as a simple substance or an elementary element and liquid noble elements for ion materials most suitable for the EHDIS. Alternatively, for this purpose, it is necessary to use noble elements from a plasma source not containing any impurities.
3 Problems related to Ion Sources
Silicon as a simple substance has a melting point of 1407.degree. C. and quite a high vapor pressure of 4.times.10.sup.-4 (torr) at the melting point. Consequently, the temperature control and the thermal vaporization control are difficult for the LMIS. Accordingly, silicon has few chances to be used as the ion material of the LMIS. On the other hand, as for germanium in the form of a simple substance, the melting point is 947.degree. C. and the vapor pressure is 1.times.10.sup.-6 (torr) at the melting point. When compared with silicon, germanium possesses quite a low melting point and a low vapor pressure thereat and hence is promising as the ion material. However, when tungsten or wolfram (W) which has been broadly employed to form an emitter of the LMIS is used for an emitter of the Si-LMIS or Ge-LMIS, tungsten is eroded in a short period of time since silicon and germanium are active substances. Resultantly, tungsten cannot function as the emitter.
In addition, refractory metals such as Ta, Mo, and Re are also eroded and dissolved in several hours when dipped into a solution of melted silicon or germanium. This leads to a problem of an extremely short life of the LMIS using such refractory metals. Moreover, a ceramic material of SiC is also attended with a problem that the ceramic completely sheds molten Si and Ge and hence cannot serve as an emitter.
Consequently, there have been few attempts to emit silicon or germanium ions from the LMIS using silicon or germanium as a simple substance. In a report of ion emission employing silicon or germanium in the form of a simple substance, the emission is conducted only quite a short period of time. There has not been reported any successful operation of such an ion sources in which emitted ions are focused into an Si-FIB or a Ge-FIB, thereby achieving a machining work of a sample. In consequence, to implement the Si-LMIS or Ge-LMIS, it is quite an important to determine a substance suitable for the emitter and the reservoir which is not eroded by the melted silicon or germanium and which guarantees to wet the emitter and the reservoir in a stable state for a long period of time.
In the known example 1, there have not been described any reason and any effect for which silicon or germanium as a single substance or an Si--Ge alloy is employed as an ion material to create ions. Neither a method of achieving a fine machining work on a semiconductor device by the FIB emitted from an ion source using silicon or germanium as a single substance nor the ion source configuration (related to emitter and reservoir materials) suitable to implement the method has been described.
Furthermore, to construct a focused ion beam (FIB) apparatus being free of impurities and using a plasma ion source, it is basically effective to generate ions from a plasma free of impurities. For this purpose, applications of a discharge mechanism not attended with impurities have been discussed. However, an identical electric field is used, in the conventional ion collecting mechanism, to extract and to accelerate ions. Namely, the ion extraction and acceleration cannot be independently controlled. In consequence, the electric field applied across the ion extracting electrode and the plasma becomes excessively high and hence possibly causes a breakdown therebetween. Furthermore, control of an ion sheath surface emitting ions is also attended with difficulty; consequently, the ions thus gathered have a weak directivity in a desired one direction, which makes it difficult to successfully develop a submicron diameter for the obtained beam.
The problems will be summarized as follows.
(1) The apparatus using Ga-FIB and the SIMS system of the prior art are attended with contamination and hence cannot be adopted in the production line of silicon semiconductor devices. As means for conducting the checks and modifications in the production line, there has not been any available check and modification apparatus which employs an ion source in place of the Ga-FIB and which is free of contamination of wafers and devices.
(2) There has been neither Si-LMIS nor Ge-LMIS adopting as an ion material silicon or germanium as an elementary material or an Si--Ge alloy, the LMIS having a long life and being capable of emitting ions with quite a high stability.
(3) For the apparatus generating a focused ion beam from noble gas ions, there has not been available a high-luminance source free of contaminating substances.
(4) There has not been any apparatus using as ion species such as Si, Ge, and noble gas ions which do not contaminate the silicon wafers and/or devices. In consequence, it is impossible to check and to modify the silicon wafers or devices without contamination thereof by such an apparatus.
Consequently, it has been long desired to devise a check or modification method removing the problem (1), to solve the problems associated with the ion sources to achieve the method, and to implement a check or modification apparatus employing the ion source in which an FIB removing the problem (4) is installed.